Colorectal cancer is the second leading cause of cancer-related deaths in the United States, largely due to invasion and metastasis to the liver and lymph nodes. Thus, a thorough understanding of invasion/metastasis is necessary for prevention and treatment of colon cancer. Over- expression of programmed cell death 4 (Pdcd4), a novel tumor suppressor, inhibits invasion and AP-1 dependent transcription in colon tumor cells. Pdcd4 expression is frequently down-regulated in colon tumor tissues as compared to adjacent normal tissues. The objective of the proposed study is to define the function of Pdcd4 in tumor invasion/metastasis and elucidate the molecular mechanisms involved in these processes. We knocked down Pdcd4 expression in colon tumor GEO and HT29 cells and found that Pdcd4 knock-down triggers morphological changes, induces invasion, and activates 2-catenin/Tcf and AP-1 dependent transcription. Our preliminary results further demonstrated that a decrease in E-cadherin expression correlates with an increase in the protein level of Slug, a transcription repressor for E-cadherin expression. We, therefore, hypothesize that Pdcd4 knock-down promotes colon tumor cell invasion/metastasis through down-regulation of E-cadherin and activation of 2-catenin/Tcf and AP-1 dependent transcription. To test our hypothesis, four Specific Aims are proposed. Aim 1: determine whether Pdcd4 functions as a metastasis inhibitor in colon tumor cells. Epithelial-to-mesenchymal transition and metastasis in mice will be assayed using Pdcd4 knock- down and over-expressed cells. Aim 2: examine whether activation of 2-catenin/Tcf dependent transcription led by E-cadherin down-regulation in Pdcd4 knock-down cells promotes invasion/metastasis. We will test activation of 2-catenin/Tcf dependent transcription by knocking down E-cadherin, reversion of the invasive/metastatic ability by inhibiting 2-catenin/Tcf dependent transcription, verification of uPAR and c-Myc as the target genes of 2-catenin/Tcf dependent transcription, and inhibition of 2-catenin/Tcf4 dependent transcription and its target gene expression by over-expressing Pdcd4. Aim 3: determine whether activation of 2-catenin/Tcf4 dependent transcription in Pdcd4 knock-down cells up-regulates MAP4K1 expression and AP-1 activity through c-Myc. We will establish the c-Myc as a regulator of MAP4K1 expression, and activation of 2- catenin/Tcf dependent transcription causing up-regulation of MAP4K1 expression, JNK activation, and AP-1 dependent transcription. Aim 4: determine whether Pdcd4 translationally inhibits transcription repressor Slug expression resulting in up-regulation of E-cadherin and suppression of invasion/metastasis. Translation inhibition and the inhibitory mechanism will be analyzed using in vitro and in vivo translation assays followed by an examination of E-cadherin expression level and the ability of invasion/metastasis in Slug knock-down and Pdcd4 expressing cells. This work will provide a comprehensive understanding of how Pdcd4 modulates colon tumor progression and holds great promise for novel interventions for preventing and treating colon cancer.